Japanese astronomers have learned that merging galaxies can contain at least a pair of active and luminous supermassive black holes. Typically, only one of the two black holes are activated like this, leading the astronomers to speculate that something unique is happening in the environment to get them going.

Astronomers theorize that collisions and mergers of small gas-rich galaxies result in the formation of massive galaxies. At the center, these objects contain a supermassive black hole (SMBHs) weighing more than one-million solar masses. When material accumulates onto a SMBH, the accretion disk surrounding it becomes extremely hot from the release of gravitational energy and it becomes very bright.

At least, not as we've defined them. In a new paper, Hawking says that one of the defining…
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Astronomers refer to this process as active galactic nucleus (AGN) activity, and it's different from the way stars generate energy via nuclear fusion reactions. They're highly compact regions which have a much higher than normal luminosity over a significant portion of the electromagnetic spectrum. They're the most luminous and persistent sources of electromagnetic radiation in the universe.

But it's not easy for astronomers to observe AGNs owing to the mess of dust and gas near these objects. To overcome this, a team of astronomers at the National Astronomical Observatory of Japan (NAOJ), led by Dr. Masatoshi Imanishi, used the Subaru Telescope's Infrared Camera and Spectrograph (IRCS) and its adaptive optics system to observe infrared luminous merging galaxies at various wavelengths.

The team observed 29 luminous gas-rich galaxies; out of these, 28 had at least one AGN (97%). These images show various aspects of the merging process:

But out of the 29 merging systems, luminous dual AGNs were detected in only four (~14%) of them. These images show emissions from multiple galaxy nuclei, and not from star-formation-related ones:

"The team's results mean that not all SMBHs in gas-rich merging galaxies are actively mass accreting, and that multiple SMBHs may have considerably different mass accretion rates onto SMBHs," noted the authors in a statement.

The finding show that the conditions near and around the SMBHs — rather than general properties of galaxies — are what dominate and regulate the mass accretion process onto SMBHs. Astronomers need to learn more about these environmental factors, particularly if they're going to improve computer simulations of galaxy mergers.